1. Field of the Invention
The present invention relates to a servo control system.
2. Description of the Prior Art
For the purpose of frequency-voltage conversion there is already known a circuit shown in FIG. 1, comprising a voltage comparator C, a mono-stable circuit M and a filter F.
Upon receipt of an AC signal as shown in FIG. 2(a) at the terminal TI, the above-mentioned circuit performs the wave-form conversion in said voltage comparator C to generate square-wave signals as shown in FIG. 2 which activates the mono-stable circuit M of which output is taken out through the filter F as an analog signal of a value corresponding to the frequency of the original signal supplied to the input terminal TI.
However, in order to obtain the signal from the output terminal TO, the frequency of the input signal has to be at least several cycles.
Also in the conventional servo control system in which the pulse signals obtained from a position detector are supplied to a low-pass filter to obtain a voltage corresponding of the speed, the output signal cannot be obtained unless at least several pulses are supplied, whereby the control being associated with a delay, thus leading to the unstable function of the system. Also when the servo motor is stopped, a false speed signal may be generated by the eventual vibration.
Besides in the conventional servo control the adjustment of the amount of speed component is conducted by the variation of the amplifying ratio of the amplifier. Thus it has been necessary to have the amplifiers or different amplifying ratios corresponding to the number of different loads and to select appropriate amplifier for example by a switch in response to the change of the load.
Furthermore, conventional servo control systems principally rely on the analog operational amplifier. For example in a conventional position servo control system shown in FIG. 3, a position signal obtained as a voltage from a potentiometer PM and a speed signal obtained from a speed generator SG are supplied, as error signals against the position instruction signal, to an operational amplifier OP for controlling a drive source M. The use of such operational amplifier has resulted in the fluctuation in the stopping position of the drive source or in the speed thereof, due to the temperature characteristics of the amplifier or the fluctuation in the power supply voltage.
Also in a servo control system wherein the drive source is controlled by an increment type encoder ENC for detecting the state of the drive source M1 as shown in FIG. 4, an operational amplifier OP2 is still used for processing the position signal which is obtained by the counting in a counter CNT of the position pulses supplied from an encoder ENC through a wave forming circuit WF followed by a digital-analog conversion in a converter DA and the speed signal obtained by a frequency-voltage conversion of said position pulses by a converter FV. Consequently the dependence on the temperature characteristics or on the fluctuation of power supply voltage is essentially same as in the above-explained case.
Furthermore, in the conventional servo control systems the speed change can be achieved in two methods. One is the conventional constant-speed servo control system shown in FIG. 5, wherein the increment type encoder ENC1 used as the speed detector generates a pulse for each displacement of a determined distance, thus providing a pulse train of a frequency corresponding to the displacing speed. Said frequency is converted by a frequency-voltage converter fV into a voltage, and the difference voltage from the instructed speed obtained in a subtractor SUB is amplified in an amplifier AMP and supplied to a drive source M2. The change of speed is achieved by the change of the instruction speed. On the other hand the variable-speed servo control is often employed as a part of the constant-position servo control, since the latter alone is frequently unable to cover the entire range due to the saturation phenomenon of the amplifier or the power supply voltage. Thus the control is effected by the constant-speed servo system while the positional error (distance to the target position) is large, and is changed to the constant-position servo system at a position close to the target. In such case there is employed a variable-speed servo control in which the instruction speed is decreased according to the value of positional error toward the target position. The other method of speed change is represented by the constant-speed servo control containing the variable-speed servo control shown in FIG. 6, wherein a switch SW makes a circuit a-c when the content of a counter CNT representing the positional error is large whereby the position-speed converting circuit PV constitutes a constant-speed servo system, thus providing instruction speeds corresponding to the positional error as shown in FIG. 7. When the positional error becomes small the switch SW makes a circuit b-c to provide a constant-position servo control. In this system, however, if the amplifier AMP1 is so designed as not to be saturated even for an extremely large positional error (G in FIG. 7), the power supplied to the drive source becomes very small in the constant-speed servo range just outside the constant position servo range (A in FIG. 7), leading to a slow start from stationary state, i.e. a longer time required to reach the instruction speed. This drawback is extremely disadvantageous for input/output apparatus for computer for which rapid start and stop are required.
Furthermore the speed generator utilized as the speed detector for example for the control of servo motor is expensive and bulky though it is capable of providing a voltage of different polarities according to the direction of rotation.
On the other hand an optical encoder can also be utilized as the speed detector as shown in FIG. 8. In this circuit the encoder ENC3 provides two signals of different phases, and the moving direction is identified by a direction detector WD from the delay between said two signals. The speed signal with direction is obtained by selecting, by means of a switch SW1 controlled by said signal of moving direction, the signal obtained by frequency-voltage conversion in a converting circuit FV3 of the output from the encoder ENC3 or the inverted signal obtained through an inverter I.
Thus, in the use of the optical encoder, it becomes necessary to effect frequency-voltage conversion and to invert the polarity of the output thereof according to the moving direction. Also the frequency-voltage converting circuit, designed to average the pulse intervals, is unable to provide instantaneous speed, thus causing delay or providing a false speed information.
Thus the above-explained circuit, if applied for example to the control of a servo motor, will lead to an extremely unstable function thereof.